Not so much a twist at this point, but it could still be interesting to see how the characters (especially Blue) react to him, and learn more about who he really is and what happened to the real Oak. It's really baffling that Game Freak scrapped the character, especially after the TCG ended up incorporating him.
Not going to lie: The hack could be superb in just about every way, but this element is pivotal for me. I lost interest in the previous Spaceworld hack because it stayed too faithful to the GS games we know. Also, Madame needs to have more than 420 BST, please. Sirfetch'd got 507, after all.
Finally, it would be great if a future update included the more recently revealed prototype Pokemon, even though we only have their front sprites. Some Kanto Pokemon could be excluded if the space restriction hasn't been resolved yet.